This invention relates to a focusing position detecting device for an optical device such as a camera. It is intended to eliminate aberrations which adversely affect the focusing position detecting operation from a focusing position detecting device which operates to detect alignment of two image parts which are obtained by dividing the image of an object, to detect focalization. Also, this invention provides an optical system which is compact and low in manufacturing cost.
A system for dividing an image into two image parts and aligning the two image parts to measure an aberration is known in the art (cf. "Optical Technique Handbook" pp. 246). A polarizing plate or an angle deflecting plate is employed as an optical system for practicing such a system. One application of the system to a focusing position detecting device is a split image prism provided for a focusing screen, for instance, in a camera.
For the focusing position detecting device, the following methods of dividing an image are available:
(1) In the first method, an image is divided into parts in the image plane.
(2) In the second method, an image forming light beam is divided into two light beams in the optical path other than the image plane.
The first method is employed for the focusing screen of a camera. In this method, different portions of an object are observed. Accordingly, the method is disadvantageous in that, if the parts of the object thus observed are different with the division line as the boundary, then focalization cannot be achieved. The second method is free from this drawback because the same portion of an object is observed.
A variety of Japanese Patent Applications have been filed for the second method: for instance, Japanese Patent Application Laid-Open No. 39544/1975, "Focus Detecting Device"; Japanese Patent Application Laid-Open No. 82419/1977, "Device for Photo-Electrically Determining where an Image is sharp"; and Japanese Patent Application Publication No. 13929/1982, "Focus Adjusting Device".
The optical system in each of these devices comprises a relay lens group for transmitting the image of an object to a sensor at the rear stage and an optical element for dividing an image forming light beam into two light beams. Some of the optical systems are added with other components, as the case may be. However, any one of the optical systems involves significant problems vis-a-vis aberration.
The first problem exists in a relay lens. In the case where an image is formed substantially in the ratio of 1:1 by the refracting system as in the prior art, it is very difficult to reduce symmetrical aberrations such as spherical aberration, an astigmatism, a curvature of field and a chromatic aberration on the axis. Accordingly, the number of lenses forming the optical system is necessarily increased.
The second problem is the location where a dividing optical element for dividing an image forming light beam into two light beams should be set. It is most effective to set the element at the place where the image of the exit pupil of the photographing lens is formed by the relay lens and other optical systems, such as, for instance, a condenser lens which is arranged close to the image plane. The arrangement of the optical element at this position is most effective in preventing reduction of the quantity of light due to vignetting. To the image of the exit pupil also, the symmetrical aberrations are harmful. Even if, when these symmetrical aberrations exist, light is uniformly applied to the pupil by an object which is uniform in brightness, the outputs of the elements of the sensor are not uniform in the case where the image of the exit pupil is within the effective diameter of the dividing optical element.